1. Field of the Invention
This invention relates to an H.sub.2 S-gettering system for use in automobile engine exhaust systems.
2. Description of the Previously Published Art
It is well-known that hydrogen sulfide (H.sub.2 S) is generated by the catalytic converters where the air-to-fuel ratio deviates significantly to the rich side while the converter remains hot. It is generally believed that a higher than usual level of H.sub.2 S released from the new breed of advanced auto exhaust catalysts is related to the higher than conventional level of rare earth oxides, especially ceria, formulated into the advanced emission control catalysts.
The mechanism for H.sub.2 S generation currently accepted involves a sequence of events. First, SO.sub.2 present in the auto exhaust stream, at about 20 ppm on the average, is oxidized to SO.sub.3 under lean conditions. This SO.sub.3 is readily captured by the catalyst where it forms sulfates of base metal oxides such as rare earth oxides and alumina. The sulfates which are stored in the catalyst are then reduced when the condition is sufficiently rich (i.e., the catalyst is exposed to a sufficiently reducing condition), to form H.sub.2 S as one of the reaction products. Such a situation arises when an automobile is suddenly decelerated or stopped after a period of high speed cruising.
To entirely eliminate or to decrease the level of H.sub.2 S emission low enough to avoid creating H.sub.2 S odor there would seem to be two possible approaches. The first would be based on a chemical modification of the existing auto exhaust catalysts, typically today's three-way catalyst (TWC) or catalysts operated under a closed-loop condition. It is, however, not an easy task to develop an auto exhaust catalyst that meets the requirements for both the long-term catalytic performance for three-way conversions as well as H.sub.2 S because the optimal catalytic performance of the existing catalysts for three-way conversions is bound to be sacrificed by a chemical modification introduced to the catalyst to meet the requirement for H.sub.2 S.
The second approach would be based on some type of auxiliary device to be used with catalytic converters for solving the H.sub.2 S problem.
3. Objects of the Invention
It is an object of this invention to add an effective H.sub.2 S gettering material to an engine exhaust system.
It is a further object of this invention to add an H.sub.2 S gettering device to an engine exhaust system, without interfering with or jeopardizing the normal function of the existing catalytic converter system.
It is a further object of this invention to select the most suitable ingredients required for an add-on H.sub.2 S trap.
It is a further object of this invention to obtain a method whereby the efficiency of this add-on H.sub.2 S trap can be increased.
It is a further object of this invention to produce an H.sub.2 S trap where an appropriate metal oxide is deposited on a support and where the metal oxide is in a partially reduced state.
These and further objects will become apparent as the description of the invention proceeds.